ABSTRACT The adaptive immune system relies on stringent immune tolerance mechanisms to ensure that self-tissues are protected from autoimmune attack. The failure and success of such immune regulation have important implications in the prevention of autoimmune diseases and the efficacy of anti-tumor immune therapies. Thus, there is great interest in defining the prevailing mechanisms that regulate T cell responses specific for self- antigens, in the hopes that these processes can be manipulated for clinical benefit. While many autoreactive T cells are thought to be purged from the conventional T (Tconv) cell repertoire by clonal deletion, substantial evidence suggests that this process is imperfect. In this regard, little is known about the nature of self-reactive T cells present in the endogenous repertoire. For example, it is unclear whether most self-specific T cells are reactive to widespread antigens or tissue-restricted antigens, and whether these cells are restricted at steady state by cell-intrinsic mechanisms such as functional inactivation or dominant mechanisms such as Treg- mediated suppression. Moreover, in the context of cancer, it has been difficult to define whether self-specific T cells contribute to the repertoire of tumor-infiltrating lymphocytes (TILs), or whether most TILs are non-specific T cells that are recruited to the tumor by TCR-independent inflammatory signals. In this proposal, we will address these unanswered questions by pursuing the following specific aims. In Aim 1, we will identify CD4+ T cell clones in the endogenous T cell repertoire that infiltrate the prostate following Treg cell ablation, and determine the nature of the self-antigens recognized by these cells. In Aim 2, we will define the tolerance mechanisms regulating these Tconv cell clones. In Aim 3, we will determine the contribution of these self- specific T cell clonotypes to the tumor infiltrate in oncogene-driven mouse prostate tumors. We will achieve these aims by testing the central hypothesis that the endogenous T cell repertoire contains a pool of self- specific Tconv cells reactive to prostate-specific antigens, and that the suppression of prostate and prostate tumor infiltration by these cells is dependent on Treg-mediated suppression. It is expected that the work outlined in this proposal will demonstrate that thymic and peripheral deletion does little to impede many self- specific Tconv clonotypes, and that Treg-mediated suppression plays a pivotal role in restricting autoimmune tissue infiltration. In addition, we anticipate that the Tconv cells infiltrating the prostate following Treg depletion will exhibit reactivity to organ-specific prostatic antigens rather than widespread self-antigens. Finally, it is expected that self-specific Tconv cells will constitute a substantial proportion of the prostate tumor-infiltrating T cell repertoire. In all, our work is expected to yield new insights in our understanding of the mechanisms underlying immune tolerance and anti-tumor immunity.